1. Field of the Invention
This invention relates to a torque wrench, more particularly to an adjustable torque limiting device for a click-type torque wrench.
2. Description of the Related Art
Referring to FIG. 1, a conventional adjustable torque wrench 10 is shown to include a shank 11, a ratchet arm 12 mounted on a front end of the shank 11, a torque assembly mounted within the shank 11 and including a roller seat 13, a spring 14, a screw pile 15, and an adjusting handle 16 mounted on a rear end of the shank 11. The ratchet arm 12 has an insert shaft 121 turnably disposed in the shank 11, a driven roller 122 disposed at an inner end of the insert shaft 121, and a ratchet head 123 disposed at an outer end of the insert shaft 121. A driving roller 131 is disposed at a front end of the roller seat 13 to be engaged with the driven roller 122. Rotation of the handle 16 results in movement of the screw pile 15 along the shank 11 so as to vary the preloaded force of the spring 14 to thereby adjust the applied torque of the torque wrench 10.
In use, a predetermined torque value is set by rotating the handle 16, and the ratchet head 123 is connected to a workpiece such as a screw fastener (not shown). The handle 16 is turned about a torque transfer axis of the ratchet head 123 to tighten the workpiece until a turning resistance between the driving and driven rollers 131,122 is overcome so that the insert shaft 121 is permitted to rotate slightly relative to the roller seat 13. At this moment, the workpiece is tightened in a desired state. However, the spring 14 is liable to bend, since there is no means for firmly supporting the spring 14. Moreover, the engagement between the driving and driven rollers 131,122 is not sufficient, so that the adjustment of the torque is not precise and turning of the workpiece is not smooth.